Conventionally, Magnetic Resonance Imaging ("MRI") has been used to produce images by exciting the nuclei of hydrogen molecules (present in water protons) in the human body. However, it has recently been discovered that polarized noble gases can produce improved images of certain areas and regions of the body which have heretofore produced less than satisfactory images in this modality. Polarized Helium 3 (".sup.3 He") and Xenon-129 (".sup.129 Xe") have been found to be particularly suited for this purpose. Unfortunately, as will be discussed further below, the polarized state of the gases are sensitive to handling and environmental conditions and, undesirably, can decay from the polarized state relatively quickly.
Hyperpolarizers are used to produce and accumulate polarized noble gases. Hyperpolarizers artificially enhance the polarization of certain noble gas nuclei (such as .sup.129 Xe or .sup.3 He) over the natural or equilibrium levels, i.e., the Boltzmann polarization. Such an increase is desirable because it enhances and increases the MRI signal intensity, allowing physicians to obtain better images of the substance in the body. See U.S. Pat. No. 5,545,396 to Albert et al., the disclosure of which is hereby incorporated herein by reference as if recited in full herein.
In order to produce the hyperpolarized gas, the noble gas is typically blended with optically pumped alkali metal vapors such as rubidium ("Rb"). These optically pumped metal vapors collide with the nuclei of the noble gas and hyperpolarize the noble gas through a phenomenon known as "spin-exchange". The "optical pumping" of the alkali metal vapor is produced by irradiating the alkali-metal vapor with circularly polarized light at the wavelength of the first principal resonance for the alkali metal (e.g., 795 nm for Rb). Generally stated, the ground state atoms become excited, then subsequently decay back to the ground state. Under a modest magnetic field (10 Gauss), the cycling of atoms between the ground and excited states can yield nearly 100% polarization of the atoms in a few microseconds. This polarization is generally carried by the lone valence electron characteristics of the alkali metal. In the presence of non-zero nuclear spin noble gases, the alkali-metal vapor atoms can collide with the noble gas atoms in a manner in which the polarization of the valence electrons is transferred to the noble-gas nuclei through a mutual spin flip "spin-exchange".
After the spin-exchange has been completed, the hyperpolarized gas is separated from the alkali metal prior to introduction into a patient to form a non-toxic or sterile composition. Unfortunately, during and after collection, the hyperpolarized gas can deteriorate or decay (lose its hyperpolarized state) relatively quickly and therefore must be handled, collected, transported, and stored carefully. The "T.sub.1 " decay constant associated with the hyperpolarized gas' longitudinal relaxation time is often used to describe the length of time it takes a gas sample to depolarize in a given container. The handling of the hyperpolarized gas is critical, because of the sensitivity of the hyperpolarized state to environmental and handling factors and the potential for undesirable decay of the gas from its hyperpolarized state prior to the planned end use, i.e., delivery to a patient. Processing, transporting, and storing the hyperpolarized gases--as well as delivery of the gas to the patient or end user--can expose the hyperpolarized gases to various relaxation mechanisms such as magnetic gradients, ambient and contact impurities, and the like.
Typically, hyperpolarized gases such as .sup.129 Xe have been collected in relatively pristine environments and transported in specialty glass containers such as rigid Pyrex.TM. containers. Flyperpolarized gas such as .sup.3 He has also been transported in Tedlar.TM. bags. Unfortunately, these conventional transport containers have produced relatively short relaxation times or can require relatively complex gas extraction systems which often leaves relatively large residual amounts of the gas in the container at the end use point.
One way of inhibiting the decay of the hyperpolarized state is presented in U.S. Pat. No. 5,612,103 to Driehuys et al. entitled Coatings for Production of Hyperpolarized Noble Gases. Generally stated, this patent describes the use of a modified polymer as a surface coating on physical systems (such as a Pyrex.TM. container) which contact the hyperpolarized gas to inhibit the decaying effect of the surface of the collection chamber or storage unit. However, there remains a need to address and refine dominant and sub-dominant relaxation mechanisms and to decrease the amount of physical systems required to deliver the hyperpolarized gas to the desired subject. Minimizing the effect of one or more of these factors can increase the life of the product by increasing the duration of the hyperpolarized state. Such an increase is desired so that the hyperpolarized product can retain sufficient polarization to allow effective imaging at delivery when transported over longer transport distances and for longer time periods from the initial polarization than has been viable previously.